Method for manufacturing solar cell

ABSTRACT

A method for manufacturing a solar cell includes performing a dry etching process to form a textured surface including a plurality of minute protrusions on a first surface of a semiconductor substrate, performing a first cleansing process for removing damaged portions of surfaces of the minute protrusions using a basic chemical and removing impurities adsorbed on the surfaces of the minute protrusions, performing a second cleansing process for removing impurities remaining or again adsorbed on the surfaces of the minute protrusions using an acid chemical after performing the first cleansing process, and forming an emitter region at the first surface of the semiconductor substrate.

This application claims priority to and the benefit of Korean PatentApplication No. 10-2012-0094199 filed in the Korean IntellectualProperty Office on Aug. 28, 2012, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to a method for manufacturing asolar cell.

2. Description of the Related Art

Recently, as existing energy sources such as petroleum and coal areexpected to be depleted, interests in alternative energy sources forreplacing the existing energy sources are increasing. Among thealternative energy sources, solar cells for generating electric energyfrom solar energy have been particularly spotlighted.

A solar cell generally includes a substrate and an emitter region, whichare formed of semiconductors of different conductive types, for example,a p-type and an n-type, and electrodes respectively connected to thesubstrate and the emitter region. A p-n junction is formed at aninterface between the substrate and the emitter region.

When light is incident on the solar cell, electrons and holes areproduced in the semiconductor parts. The electrons move to the n-typesemiconductor part, and the holes move to the p-type semiconductor partunder the influence of the p-n junction of the semiconductor parts.

Then, the electrons and the holes are collected by the differentelectrodes respectively connected to the n-type semiconductor part andthe p-type semiconductor part.

SUMMARY OF THE INVENTION

In one aspect, there is a method for manufacturing a solar cellincluding performing a dry etching process to form a textured surfaceincluding a plurality of minute protrusions on a first surface of asemiconductor substrate, performing a first cleansing process forremoving damaged portions of surfaces of the minute protrusions using abasic chemical and removing impurities adsorbed on the surfaces of theminute protrusions, performing a second cleansing process for removingimpurities remaining or again adsorbed on the surfaces of the minuteprotrusions using an acid chemical after performing the first cleansingprocess, and forming an emitter region at the first surface of thesemiconductor substrate.

The dry etching process includes a reaction ion etching method.

The basic chemical may be formed by mixing ultrapure water and a basicmaterial having a hydroxyl radical (OH). Further, the basic chemical mayadditionally contain hydrogen peroxide. The acid chemical may be formedby mixing ultrapure water, hydrogen chloride, and hydrogen peroxide.Alternatively, the acid chemical may be formed by mixing ultrapurewater, hydrogen chloride, and hydrogen fluoride. The method may furtherinclude again cleansing the surfaces of the minute protrusions using adiluted acid chemical between the first cleansing process and the secondcleansing process and/or after the second cleansing process. The dilutedacid chemical may be formed by mixing ultrapure water and hydrogenfluoride.

The forming of the emitter region includes injecting impurities of afirst conductive type into the first surface of the semiconductorsubstrate using an ion implantation method or a thermal diffusionmethod.

The method may further include forming a second textured surfaceincluding a plurality of minute protrusions on a second surface oppositethe first surface of the semiconductor substrate, and locally forming aback surface field region at the second surface of the semiconductorsubstrate.

The forming of the back surface field region includes injectingimpurities of a second conductive type opposite the first conductivetype into the second surface of the semiconductor substrate using theion implantation method or the thermal diffusion method.

The method may further include forming a first dielectric layer on thesecond surface of the semiconductor substrate, simultaneously forming asecond dielectric layer on the emitter region and on the firstdielectric layer positioned on the second surface of the semiconductorsubstrate, forming a third dielectric layer on the second dielectriclayer positioned on the emitter region, and forming a first electrodepart connected to the emitter region and a second electrode partconnected to the back surface field region.

The first dielectric layer and the third dielectric layer may be formedby depositing hydrogenated silicon nitride at a thickness of about 70 nmto 100 nm. The second dielectric layer may be formed by depositingaluminum oxide at a thickness of about 5 nm to 15 nm, and aluminum oxidemay be deposited using an atomic layer deposition method.

The back surface field region may be formed in the same pattern as aplurality of finger electrodes of the second electrode part.

According to the above-described characteristics, embodiments of theinvention perform the first cleansing process for removing the damagedportions of the surfaces of the minute protrusions using the basicchemical and at the same time removing impurities adsorbed on thesurfaces of the minute protrusions, and perform the second cleansingprocess for removing impurities again adsorbed on the surfaces of theminute protrusions using the acid chemical. Then, the embodiments of theinvention form the emitter region at the first surface of thesemiconductor substrate.

Accordingly, the impurities adsorbed on the surfaces of the minuteprotrusions in a bath may be efficiently removed.

When the surfaces of the minute protrusions are again cleansed using thediluted acid chemical after the second cleansing process, an oxide layeron the surfaces of the minute protrusions after the second cleansingprocess may be removed, and impurities remaining in the surfaces of theminute protrusions may be again removed. Therefore, the cleansingoperation may be more efficiently performed.

Because the emitter region is formed after performing the first andsecond cleansing processes, the emitter region may be stably formed.

Accordingly, current characteristics of the solar cell may be improved,and the damaged portions of the surfaces of the minute protrusions andthe impurities adsorbed on the surfaces of the minute protrusions may beefficiently removed. Hence, efficiency of the solar cell may beimproved.

Furthermore, because an anti-reflection layer and a passivation layer,each of which has a multi-layered structure, are respectively formed onthe first and second surfaces of the semiconductor substrate, areflection amount of light is reduced, and a surface passivation effectis obtained at the surface of the semiconductor substrate. Hence, theefficiency of the solar cell may be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a partial perspective view of a solar cell according to anexample embodiment of the invention;

FIG. 2 is an enlarged view of a main part of FIG. 1;

FIG. 3 is a conceptual diagram illustrating a ratio of a surface area toa real area of a second textured surface;

FIGS. 4A to 4H sequentially illustrate a method for manufacturing asolar cell according to an example embodiment of the invention;

FIG. 5 is a block diagram illustrating a cleansing method according toan example embodiment of the invention;

FIGS. 6A and 6B are photographs of minute protrusions taken by amicroscope after the minute protrusions are formed using a reactive ionetching method;

FIGS. 7A and 7B are photographs of minute protrusions taken by amicroscope after a first cleansing process and a second cleansingprocess are performed using an acid chemical; and

FIGS. 8A and 8B are photographs of minute protrusions taken by amicroscope after a first cleansing process using a basic chemical and asecond cleansing process using an acid chemical are performed.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts. It will be paidattention that detailed description of known arts will be omitted if itis determined that the known arts can obscure the embodiments of theinvention.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. It will be understood that when an elementsuch as a layer, film, region, or substrate is referred to as being “on”another element, it can be directly on the other element or interveningelements may also be present.

In contrast, when an element is referred to as being “directly on”another element, there are no intervening elements present.

Further, it will be understood that when an element such as a layer,film, region, or substrate is referred to as being “entirely” on otherelement, it may be on the entire surface of the other element and maynot be on a portion of an edge of the other element.

Example embodiments of the invention will be described with reference toFIGS. 1 to 8B.

A solar cell according to an example embodiment of the invention isdescribed in detail with reference to FIG. 1.

As shown in FIG. 1, a solar cell according to an example embodiment ofthe invention includes a substrate 110, an emitter region 121 positionedat a front surface (or a first surface) of the substrate 110, a firstdielectric layer part 130 positioned on the emitter region 121, a seconddielectric layer part 190 positioned on a back surface (or a secondsurface) opposite the front surface of the substrate 110, a frontelectrode part (or a first electrode part) 140 which is positioned onthe front surface of the substrate 110, is connected to the emitterregion 121, and includes a plurality of front electrodes (or a pluralityof first finger electrodes) 141 and a plurality of front bus bars (or aplurality of first bus bars) 142, a back electrode part (or a secondelectrode part) 150 which is positioned on the back surface of thesubstrate 110 and includes a plurality of back electrodes (or aplurality of second finger electrodes) 151 and a plurality of back busbars (or a plurality of second bus bars) 152, and a plurality of backsurface field regions 172 which are positioned under the plurality ofback electrodes 151 and the plurality of back bus bars 152 at the backsurface of the substrate 110.

In the embodiment of the invention, light is incident on at least one ofthe front surface and the back surface of the substrate 110.

The substrate 110 is a semiconductor substrate formed of a semiconductorsuch as first conductive type silicon, for example, n-type silicon,though not required. The semiconductor used in the substrate 110 is acrystalline semiconductor formed of single crystal silicon. The n-typesubstrate 110 is doped with impurities of a group V element such asphosphorus (P), arsenic (As), and antimony (Sb).

A texturing process is performed on the front surface of the substrate110 to form a first textured surface corresponding to an uneven surfacehaving a plurality of first protrusions 11 or having unevencharacteristics. In this instance, the emitter region 121 and the firstdielectric layer part 130 positioned on the front surface of thesubstrate 110 each have an uneven surface.

Each of the first protrusions 11 has a pyramid shape.

In the embodiment disclosed herein, the size, i.e., a maximum width ‘a’and a maximum height ‘b’ of each of the first protrusions 11 may beabout 5 μm to 15 μm. An aspect ratio ‘b/a.’ of each first protrusion 11may be about 1.0 to 1.5.

Because the plurality of first protrusions 11 are positioned on thefront surface of the substrate 110, an incident area of the substrate110 increases and a light reflectance decreases due to a plurality ofreflection operations resulting from each first protrusion 11. Hence, anamount of light incident on the substrate 110 increases, and theefficiency of the solar cell is improved.

As shown in FIG. 1, the plurality of first protrusions 11 may be formedon the back surface of the substrate 110. Alternatively, the firstprotrusions 11 may not be formed on the back surface of the substrate110.

The maximum width ‘a’ of the first protrusion 11 may be determinedwithin the range of about 5 μm to 15 μm. The size of the firstprotrusion 11 increases as the maximum width ‘a’ increases based onproperties of crystalline silicon forming the substrate 110, and viceversa.

Accordingly, when the size of the first protrusion 11 is optimized sothat the maximum width ‘a’ of the first protrusion 11 is about 5 μm to15 μm and the aspect ratio ‘b/a’ of the first protrusion 11 is about 1.0to 1.5, an optimum light path of incident light may be obtained.

As shown in FIG. 2, a plurality of minute protrusions 111 (hereinafterreferred to as ‘second protrusions’) are positioned on the surface ofeach first protrusion 11, and thus a second textured surface is formedon the surface of each first protrusion 11.

The size (i.e., a maximum width and a maximum height) of each of thesecond protrusions 111 formed on the surface of each first protrusion 11is less than the size of the first protrusion 11.

For example, the size, i.e., the maximum width and the maximum height ofthe second protrusion 111 may be several hundreds of nanometers, forexample, about 300 nm to 600 nm.

When the second protrusions 111 are formed on the surface of the firstprotrusion 11, the incident area of the substrate 110 further increases.Further, because the reflection operation of light is repeatedlyperformed due to the second protrusions 111, an amount of light incidenton the substrate 110 further increases.

As described above, the surface of the substrate 110 is formed as thefirst textured surface having the plurality of first protrusions 11, andthe surface of each first protrusion 11 is formed as the second texturedsurface having the plurality of second protrusions 111. Thus, thesurface of the substrate 110 has the double textured surface. Hence,light having a wavelength of about 300 nm to 1,100 nm has a lowreflectance (for example, average weighted reflectance) of about 1% to10%.

The second protrusions 111 are described in detail below.

As described above, the plurality of second protrusions 111 are formedon the surface of each first protrusion 11, and each second protrusion111 has a maximum width and a maximum height of about 300 nm to 600 nm.

In the embodiment disclosed herein, the maximum width of the secondprotrusion 111 is a distance between valleys of the second protrusion111 in the same manner as the maximum width of the first protrusion 11.Further, the maximum height of the second protrusion 111 is a shortestdistance ranging from a virtual line connecting the valleys of thesecond protrusion 111 to a peak of the second protrusion 111 in the samemanner as the maximum height of the first protrusion 11.

In a vertical cross section of the second protrusions 111, a ratio a1/b1of a length a1 of a virtual line connecting vertexes of the secondprotrusions 111 to a length b1 of a straight line connecting a startpoint SP and a finish point FP of the virtual line is about 1.1 to 1.3.

FIG. 2 illustrates the measurement of the ratio a1/b1 of the nine secondprotrusions 111. However, the ratio a1/b1 may be measured using thethree or more second protrusions 111. It is preferable, but notrequired, that at least five second protrusions 111 are used so as tosecure the reliability of the measurement.

According to an experiment of the present inventors, when the ratioa1/b1 was greater than about 1.3, the size, i.e., the maximum width andthe maximum height of each second protrusion 111 were about 500 nm to1,000 nm. Namely, the sizes of the second protrusions 111 werenon-uniform and thus entirely had low uniformity.

Further, when the ratio a1/b1 was less than about 1.1, the size of eachsecond protrusion 111 was equal to or less than about 200 nm. Namely,the sizes of the second protrusions 111 were uniform and thus entirelyhad good uniformity.

The uniformity of the second protrusions 111 when the ratio a1/b1 wasless than about 1.1 was more excellent than the uniformity of the secondprotrusions 111 when the ratio a1/b1 was greater than about 1.3.

On the other hand, a reflectance of light when the ratio a1/b1 wasgreater than about 1.3 was equal to or less than about 7%, and areflectance of light when the ratio a1/b1 was less than about 1.1 wasequal to or greater than about 10%.

As described above, the reflectance of the second textured surfaceincreases and decreases in inverse proportion to the ratio a1/b1. Thisreason is that the sizes of the second protrusions 111 decrease as theratio a1/b1 becomes close to 1, and thus the reflectance of the secondtextured surface increases.

As described above, when the ratio a1/b1 is greater than about 1.3, thereflectance is low. Thus, a conversion efficiency of the solar cell canbe improved. However, the sizes of the second protrusions 111 having theratio a1/b1 greater than about 1.3 are non-uniform and entirely have thelow uniformity, as compared with the second protrusions 111 having theratio a1/b1 less than about 1.1. Therefore, when the ratio a1/b1 isgreater than about 1.3, a recombination rate of electrons and holesincreases.

In addition, a current path increases, and a dead area increases. Thus,a loss of current greatly increases because of the above reasons.Therefore, it is preferable, but not required, that the second texturedsurface is formed so that the ratio a1/b1 is set to be equal to or lessthan about 1.3, so as to improve the conversion efficiency of the solarcell.

Further, the second textured surface having the ratio a1/b1 less thanabout 1.1 may suppress the problem generated in the second texturedsurface having the ratio a1/b1 greater than about 1.3. However, becausethe reflectance of the second textured surface having the ratio a1/b1less than about 1.1 greatly increases, a short circuit current densityincreases as compared with the second textured surface having the ratioa1/b1 greater than about 1.3. Hence, the conversion efficiency of thesolar cell is reduced.

Accordingly, it is preferable, but not required, that the secondtextured surface is formed so that the ratio a1/b1 is set to be equal toor greater than about 1.1, so as to improve the conversion efficiency ofthe solar cell.

According to the above description, it may be preferable, but notrequired, that the second protrusions 111 of the second textured surfacehave the ratio a1/b1 of about 1.1 to 1.3 and each have the size of about300 nm to 600 nm.

As described above, when the ratio a1/b1 of the second protrusions 111of the second textured surface is about 1.1 to 1.3, a ratio of a surfacearea to a real area of the second textured surface in a unit area, forexample, the area of 10 μm×10 μm of the second textured surface is about2 to 2.5. The size of the unit area may be changed.

In the embodiment disclosed herein, the surface area is a sum (forexample, a sum of surface areas of triangles A, B, C, D, E, F, G, H, I,and J shown in FIG. 3) of surface areas of the second protrusions 111formed on the second textured surface of the unit area. The real area isa projection area (indicated by ‘S’ of FIG. 3) viewed from a verticaldirection of the surface of the substrate 110.

The emitter region 121 positioned at the front surface of the substrate110 is an impurity region doped with impurities of a second conductivetype (for example, p-type) opposite the first conductive type (forexample, n-type) of the substrate 110. Thus, the emitter region 121 ofthe second conductive type forms a p-n junction along with the substrate110 of the first conductive type.

In the embodiment of the invention, a sheet resistance of the emitterregion 121 may be equal to or less than about 150 Ω/sq., preferably,about 70 Ω/sq. to 80 Ω/sq.

Regarding carriers, for example, electrons and holes produced by lightincident on the substrate 110, the electrons and the holes respectivelymove to the n-type semiconductor and the p-type semiconductor by abuilt-in potential difference resulting from the p-n junction betweenthe substrate 110 and the emitter region 121. Thus, when the substrate110 is of the n-type and the emitter region 121 is of the p-type, theelectrons and the holes move to the substrate 110 and the emitter region121, respectively.

When the emitter region 121 is of the p-type, the emitter region 121 maybe formed by doping the substrate 110 with impurities of a group IIIelement such as boron (B), gallium (Ga), and indium (In). In thisinstance, the emitter region 121 may be formed using an ion implantationmethod, among other methods.

The first dielectric layer part 130 includes a second dielectric layer131 positioned on the emitter region 121 and a third dielectric layer132 positioned on the second dielectric layer 131.

In the embodiment of the invention, the second dielectric layer 131 maybe formed of aluminum oxide (Al₂O₃), and the third dielectric layer 132may be formed of hydrogenated silicon nitride (SiNx:H).

The second dielectric layer 131 formed of aluminum oxide (Al₂O₃) mayhave a thickness of about 5 nm to 15 nm and a refractive index of about1.1 to 1.6. The third dielectric layer 132 formed of hydrogenatedsilicon nitride (SiNx:H) may have a thickness of about 70 nm to 100 nmand a refractive index of about 2.0 to 2.2.

In this instance, because the refractive index of the second dielectriclayer 131 adjacent to the substrate 110 is less than the refractiveindex of the third dielectric layer 132 adjacent to an air, ananti-reflection effect is reduced due to the refractive index of thesecond dielectric layer 131. It is preferable, but not required, thatthe thickness of the second dielectric layer 131 is much less than thethickness of the third dielectric layer 132, so as to prevent areduction in the anti-reflection effect.

The second dielectric layer 131 formed of aluminum oxide (Al₂O₃) ispositioned on the front surface of the substrate 110, i.e., directly onthe emitter region 121 positioned at the front surface of the substrate110.

In general, aluminum oxide (Al₂O₃) has negative fixed charges.

Thus, positive fixed charges (i.e., holes) are drawn to the p-typeemitter region 121 and electrons are pushed to the back surface of thesubstrate 110 by the second dielectric layer 131 formed of aluminumoxide (Al₂O₃), which is positioned on the p-type emitter region 121 andhas negative fixed charges. Namely, a field passivation effect isobtained.

Accordingly, an amount of holes moving to the emitter region 121 furtherincreases by the second dielectric layer 131 formed of aluminum oxide(Al₂O₃), and an amount of electrons moving to the emitter region 121decreases by the second dielectric layer 131. Hence, a recombination ofelectrons and holes at and around the emitter region 121 is reduced.

Oxygen (O) contained in aluminum oxide (Al₂O₃) for forming the seconddielectric layer 131 moves to the surface of the substrate 110 abuttingon the second dielectric layer 131, thereby performing a passivationfunction for converting a defect, for example, dangling bonds existingat and around the surface of the substrate 110 into stable bonds.

It is preferable, but not required, that the second dielectric layer 131formed of aluminum oxide (Al₂O₃) is formed using an atomic layerdeposition (ALD) method having a good step coverage.

As described above, the second textured surface as well as the firsttextured surface are formed on the front surface of the substrate 110.Therefore, a roughness of the front surface (i.e., the surface of theemitter region 121) of the substrate 110 abutting on the seconddielectric layer 131 of the first dielectric layer part 130 is greaterthan a roughness of the substrate having only the first texturedsurface.

If the second dielectric layer 131 is formed directly on the emitterregion 121 using a deposition method such as a plasma enhanced chemicalvapor deposition (PECVD) method, the second dielectric layer 131 may notbe normally coated on the first and second protrusions 11 and 111.Hence, a non-formation area of the second dielectric layer 131 mayincrease in the first and second textured surfaces of the substrate 110.

In this instance, a surface passivation effect may not be generated inthe non-formation area of the second dielectric layer 131. As a result,an amount of carriers lost at and around the surface of the substrate110 may increase.

On the other hand, when the second dielectric layer 131 is formeddirectly on the emitter region 121 using the atomic layer depositionmethod having the good step coverage as in the embodiment of theinvention, the second dielectric layer 131 is normally formed on thefirst and second protrusions 11 and 111. Hence, the non-formation areaof the second dielectric layer 131 decreases in the first and secondtextured surfaces of the substrate 110.

Accordingly, because a formation area of the second dielectric layer 131increases in the first and second textured surfaces of the substrate110, the surface passivation effect using the second dielectric layer131 is improved. Hence, an amount of carriers lost at and around thesurface of the substrate 110 decreases, and the efficiency of the solarcell is improved.

The third dielectric layer 132 formed of hydrogenated silicon nitride(SiNx:H) is formed directly on the second dielectric layer 131positioned on the front surface of the substrate 110.

Hydrogen (H) contained in hydrogenated silicon nitride (SiNx:H) forforming the third dielectric layer 132 moves to the surface of thesubstrate 110 via the second dielectric layer 131, thereby performing apassivation function at and around the surface of the substrate 110.

Accordingly, an amount of carriers lost by the defect at and around thesurface of the substrate 110 further decreases by the passivationfunction resulting from the third dielectric layer 132 as well as thesecond dielectric layer 131.

As described above, the first dielectric layer part 130 positioned onthe front surface of the substrate 110 has a double layeredanti-reflection structure including the second dielectric layer 131formed of aluminum oxide (Al₂O₃) and the third dielectric layer 132formed of hydrogenated silicon nitride (SiNx:H).

Accordingly, the field passivation effect resulting from the fixedcharges of the second dielectric layer 131 and the surface passivationeffect resulting from the second and third dielectric layers 131 and 132are additionally obtained in addition to the anti-reflection effect oflight using changes in the refractive indexes of the second and thirddielectric layers 131 and 132.

When the thickness of the second dielectric layer 131 formed of aluminumoxide is equal to or greater than about 5 nm, the aluminum oxide layer131 is more uniformly formed and the field passivation effect resultingfrom the fixed charges of the second dielectric layer 131 is more stablyobtained through the stable generation of the fixed charges of thesecond dielectric layer 131. When the thickness of the second dielectriclayer 131 is equal to or less than about 15 nm, manufacturing time andcost of the second dielectric layer 131 are reduced without a reductionin the anti-reflection effect resulting from the refractive indexes ofthe second and third dielectric layers 131 and 132.

When the thickness of the third dielectric layer 132 formed ofhydrogenated silicon nitride is equal to or greater than about 70 nm,the silicon nitride layer 132 is more uniformly formed and the surfacepassivation effect using hydrogen (H) is more stably obtained. When thethickness of the third dielectric layer 132 is equal to or less thanabout 100 nm, the field passivation effect resulting from hydrogenatedsilicon nitride having positive fixed charges is not reduced. Further,manufacturing time and cost of the third dielectric layer 132 arereduced.

Each of the back surface field regions 172 positioned at the backsurface of the substrate 110 is a region that is more heavily doped thanthe substrate 110 with impurities of the same first conductive type (forexample, the n-type) as the substrate 110.

The back surface field regions 172 abut on the back electrodes 151 andthe back bus bars 152 positioned on the back surface of the substrate110 and are locally positioned at the back surface of the substrate 110.

The fact that the back surface field region 172 is locally positioned atthe back surface of the substrate 110 means that the back surface fieldregion 172 is positioned only at the back surface of the substrate 110at a position corresponding to at least one of the back electrode 151and the back bus bar 152.

Hence, the back surface field region 172 is not positioned between theadjacent back electrodes 151, between the back electrode 151 and theback bus bar 152 which are positioned adjacent to each other, andbetween the adjacent back bus bars 152.

A potential barrier is formed by a difference between impurityconcentrations of a first conductive type region (for example, an n-typeregion) of the substrate 110 and the back surface field regions 172.Hence, the potential barrier prevents or reduces holes from moving tothe back surface field regions 172 used as a moving path of electronsand makes it easier for electrons to move to the back surface fieldregions 172.

Thus, the back surface field regions 172 reduce an amount of carrierslost by a recombination and/or a disappearance of the electrons and theholes at and around the back surface of the substrate 110 and acceleratea movement of desired carriers (for example, electrons), therebyincreasing an amount of carriers moving to the back electrode part 150.

Because the impurity concentration of the back surface field regions 172is higher than the impurity concentration of the substrate 110, theconductivity of the back surface field regions 172 abutting on the backelectrode part 150 is greater than the conductivity of the substrate110. Hence, a movement of carriers from the back surface field regions172 to the back electrode part 150 is easily carried out.

The second dielectric layer part 190 includes a first dielectric layer191 positioned directly on the back surface of the substrate 110 and asecond dielectric layer 131 positioned directly on the first dielectriclayer 191.

The first dielectric layer 191 may be formed of hydrogenated siliconnitride (SiNx:H), and the second dielectric layer 131 may be formed ofaluminum oxide (Al₂O₃) as described above.

In this instance, the first dielectric layer 191 may be formed of thesame material as the third dielectric layer 132 and thus may have thesame characteristics, for example, thickness, properties, component,composition (or composition ratio), refractive index, etc. as the thirddielectric layer 132.

More specifically, the first dielectric layer 191 formed of hydrogenatedsilicon nitride (SiNx:H) may have a thickness of about 70 nm to 100 nmand a refractive index of about 2.0 to 2.2 in the same manner as thethird dielectric layer 132. The second dielectric layer 131 formed ofaluminum oxide (Al₂O₃) may have a thickness of about 5 nm to 15 nm and arefractive index of about 1.1 to 1.6.

Because the first dielectric layer 191 formed of hydrogenated siliconnitride (SiNx:H) is positioned on the back surface field regions 172positioned directly on the back surface of the substrate 110, thesurface passivation function using hydrogen (H) is performed. Hence, anamount of carriers lost by the defect at and around the back surface ofthe substrate 110 decreases.

Hydrogenated silicon nitride (SiNx:H) has the characteristic of positivefixed charges opposite aluminum oxide (Al₂O₃).

In the embodiment disclosed herein, the substrate 110 is of the n-type,and the first dielectric layer 191 formed of hydrogenated siliconnitride (SiNx:H) is formed directly on the back surface of the substrate110. Hence, because negative charges (i.e., electrons) moving to thefirst dielectric layer 191 have the polarity opposite the firstdielectric layer 191 having the characteristic of positive charges, thenegative charges (i.e., electrons) are drawn to the first dielectriclayer 191 due to the positive polarity of the first dielectric layer191.

Further, positive charges (i.e., holes) having the same polarity as thefirst dielectric layer 191 are pushed to the front surface of thesubstrate 110 opposite the first dielectric layer 191 because of thepositive polarity of the first dielectric layer 191.

Hence, when hydrogenated silicon nitride (SiNx:H) is deposited directlyon the back surface of the n-type substrate 110 to form the firstdielectric layer 191, an amount of electrons moving to the back surfaceof the substrate 110 further increases because of the influence ofpositive fixed charges. Further, the recombination of the electrons andthe holes at and around the back surface of the substrate 110 isreduced.

The second dielectric layer 131, which is formed on the first dielectriclayer 191 using aluminum oxide (Al₂O₃), prevents hydrogen (H) containedin the first dielectric layer 191 from moving to the back electrode part150 opposite the front surface of the substrate 110 because of heatapplied when the solar cell is manufactured. As a result, the surfacepassivation effect of the back surface of the substrate 110 usinghydrogen (H) contained in the first dielectric layer 191 is improved.

As described above, the second dielectric layer part 190 including thefirst dielectric layer 191 formed of hydrogenated silicon nitride(SiNx:H) and the second dielectric layer 131 formed of aluminum oxide(Al₂O₃) has a double layered anti-reflection structure at the backsurface of the substrate 110, in the same manner as the double layeredanti-reflection structure of the first dielectric layer part 130positioned on the front surface of the substrate 110. Hence, the surfacepassivation effect of the back surface of the substrate 110 is improved.

It is preferable, but not required, that the thickness of the firstdielectric layer 191 is greater than the thickness of the seconddielectric layer 131 positioned on the first dielectric layer 191, sothat the second dielectric layer 131 having the strong negative fixedcharges does not adversely affect the first dielectric layer 191 havingthe positive fixed charges. Further, the thickness of the firstdielectric layer 191 may be greater than the thickness of the thirddielectric layer 132 on the front surface of the substrate 110.

Accordingly, if necessary, the thickness of the third dielectric layer132 positioned on the front surface of the substrate 110 may bedifferent from the thickness of the first dielectric layer 191positioned on the back surface of the substrate 110. In this instance,the third dielectric layer 132 positioned on the front surface of thesubstrate 110 may have the thickness of about 90 nm, and the firstdielectric layer 191 positioned on the back surface of the substrate 110may have the thickness of about 100 nm.

When light is incident on the back surface of the substrate 110, arefractive index from an air to the substrate 110 increases. Therefore,a reflection amount of light incident on the back surface of thesubstrate 110 decreases, and an amount of light incident into thesubstrate 110 decreases. As described above, when light is incident onthe back surface of the substrate 110, the second dielectric layer part190 may serve as an anti-reflection layer.

The plurality of front electrodes 141 of the front electrode part 140are connected to the emitter region 121, and the plurality of front busbars 142 of the front electrode part 140 are connected to the frontelectrodes 141 as well as the emitter region 121.

The front electrodes 141 are electrically and physically connected tothe emitter region 121 and are separated from one another. The frontelectrodes 141 extend parallel to one another in a fixed direction. Thefront electrodes 141 collect carriers (for example, holes) moving to theemitter region 121.

The front bus bars 142 are electrically and physically connected to theemitter region 121 and extend parallel to one another in a directioncrossing the front electrodes 141.

The front bus bars 142 have to collect not only carriers (for example,holes) moving from the emitter region 121 but also carriers collected bythe front electrodes 141 crossing the front bus bars 142 and have tomove the collected carriers in a desired direction. Thus, a width ofeach front bus bar 142 is greater than a width of each front electrode141.

In the embodiment of the invention, the front bus bars 142 arepositioned on the same level layer as the front electrodes 141 and areelectrically and physically connected to the front electrodes 141 atcrossings of the front electrodes 141 and the front bus bars 142.

Accordingly, as shown in FIG. 1, the plurality of front electrodes 141have a stripe shape extending in a transverse (or longitudinal)direction, and the plurality of front bus bars 142 have a stripe shapeextending in a longitudinal (or transverse) direction. Hence, the frontelectrode part 140 has a lattice shape on the front surface of thesubstrate 110.

The front bus bars 142 are connected to an external device and outputthe collected carriers to the external device.

The front electrode part 140 including the front electrodes 141 and thefront bus bars 142 is formed of at least one conductive material, forexample, silver (Ag).

The plurality of back electrodes 151 of the back electrode part 150 arepositioned on the back surface field regions 172 and directly abut onthe back surface field regions 172. The back electrodes 151 areseparated from one another and extend in a fixed direction in the samemanner as the front electrodes 141.

In this instance, the back electrodes 151 extend in the same directionas the front electrodes 141. The back electrodes 151 collect carriers(for example, electrons) moving to the back surface field regions 172.

The plurality of back bus bars 152 of the back electrode part 150 arepositioned on the back surface field regions 172 and abut on the backsurface field regions 172. The back bus bars 152 extend parallel to oneanother in a direction crossing the back electrodes 151.

In this instance, the back bus bars 152 extend in the same direction asthe front bus bars 142. The back bus bars 152 may be positioned oppositethe front bus bars 142 with the substrate 110 interposed between them.

The back bus bars 152 collect carriers (for example, electrons)collected by the back electrodes 151 crossing the back bus bars 152 andmove the collected carriers in a desired direction. Thus, a width ofeach back bus bar 152 is greater than a width of each back electrode151.

The back bus bars 152 are positioned on the same level layer as the backelectrodes 151 and are electrically and physically connected to the backelectrodes 151 at crossings of the back electrodes 151 and the back busbars 152.

Thus, the back electrode part 150 has a lattice shape on the backsurface of the substrate 110 in the same manner as the front electrodepart 140.

The back electrodes 151 and the back bus bars 152 may contain the sameconductive material, for example, silver (Ag) as the front electrodes141 and the front bus bars 142. Alternatively, the back electrode part150 may be formed of a material different from the front electrode part140, and the back electrodes 151 may be formed of a material differentfrom the back bus bars 152.

As described above, in the embodiment of the invention, the back surfacefield regions 172 are positioned under the back electrodes 151 and theback bus bars 152 and extend along the back electrodes 151 and the backbus bars 152.

Hence, the back surface field regions 172 are locally positioned at theback surface of the substrate 110 and have a lattice shape in the samemanner as the back electrode part 150. Thus, as described above, anon-formation portion of the back surface field regions 172 exists atthe back surface of the substrate 110.

In the embodiment of the invention, the number of front electrode 141positioned on the front surface of the substrate 110, on which the mostof light is incident, is less than the number of back electrode 151positioned on the back surface of the substrate 110, on which a smalleramount of light than the front surface of the substrate 110 is incident.Thus, a distance between the two adjacent front electrodes 141 isgreater than a distance between the two adjacent back electrodes 151.

As described above, because the front electrode part 140 and the backelectrode part 150 contain a metal material such as silver (Ag), thefront electrode part 140 and the back electrode part 150 do not transmitlight.

Accordingly, because the distance between the front electrodes 141positioned on the front surface of the substrate 110 is greater than thedistance between the back electrodes 151, a reduction in an incidentarea of light at the front surface of the substrate 110 is prevented bythe front electrodes 141. Hence, an amount of light incident on thefront surface of the substrate 110 increases.

In another embodiment, the front bus bars 142, the back bus bars 152, orboth may be omitted.

In the embodiment of the invention, at least one of the front electrodepart 140 and the back electrode part 150 may be formed using a platingmethod.

When at least one of the front electrode part 140 and the back electrodepart 150 is formed using the plating method, at least one of the frontelectrode part 140 and the back electrode part 150 may have asingle-layered structure as in the embodiment of the invention.Alternatively, at least one of the front electrode part 140 and the backelectrode part 150 may have a multi-layered structure such as adouble-layered structure and a triple-layered structure. When at leastone of the front electrode part 140 and the back electrode part 150formed using the plating method has the single-layered structure, atleast one of the front electrode part 140 and the back electrode part150 may be formed of silver (Ag).

When at least one of the front electrode part 140 and the back electrodepart 150 formed using the plating method has the double-layeredstructure, a lower layer (or a first layer), which abuts on the emitterregion 121 (i.e., a second conductive type region of the substrate 110)or abuts on the back surface field regions 172 (i.e., a heavily dopedregion of the substrate 110 doped with impurities of the firstconductive type), may be formed of nickel (Ni), and an upper layer (or asecond layer) on the lower layer may be formed of silver (Ag).

When at least one of the front electrode part 140 and the back electrodepart 150 formed using the plating method has the triple-layeredstructure, a lower layer (or a first layer) abutting on the emitterregion 121 or the back surface field regions 172 may be formed of nickel(Ni), a middle layer (or a second layer) on the lower layer may beformed of copper (Cu), and an upper layer (or a third layer) on themiddle layer may be formed of silver (Ag) or tin (Sn).

When at least one of the front electrode part 140 and the back electrodepart 150 formed using the plating method has the double-layeredstructure, a thickness of the lower layer may be about 0.5 μm to 1 μm,and a thickness of the upper layer may be about 5 μm to 10 μm.

When at least one of the front electrode part 140 and the back electrodepart 150 formed using the plating method has the triple-layeredstructure, each of the lower layer and the upper layer may have athickness of about 0.5 μm to 1 μm, and a thickness of the middle layermay be about 5 μm to 10 μm.

In this instance, the lower layer is to reduce a contact resistancebetween the lower layer and the emitter region 121 abutting on the lowerlayer or between the lower layer and the back surface field regions 172abutting on the lower layer, thereby improving contact characteristics.The middle layer may be formed of a cheap material with the goodconductivity, for example, copper (Cu) in consideration of costreduction.

If the middle layer is formed of copper (Cu), the lower layer underlyingthe middle layer may prevent copper (Cu), which is smoothly bonded tosilicon (Si), from serving as an impurity region, which is penetrated(absorbed) in the emitter region 121 or the back surface field regions172 formed of silicon (Si) to thereby prevent the movement of carriers.

The upper layer prevents the oxidation of the layer (for example, thelower layer or the middle layer) underlying the upper layer and improvesan adhesive strength between the layer (for example, the lower layer orthe middle layer) and a conductive film, for example, a ribbonpositioned on the upper layer.

As described above, at least one of the front electrode part 140 and theback electrode part 150 formed using the plating method may have thedouble-layered structure or the triple-layered structure, and the lowerlayer may be formed of nickel (Ni). In this instance, nickel silicidecompounds exist between the lower layer and the emitter region 121 orbetween the lower layer and the back surface field regions 172 becauseof a bond between nickel (Ni) and silicon (Si) of the emitter region 121(i.e., the second conductive type region of the substrate 110) or a bondbetween nickel (Ni) and silicon (Si) of the back surface field regions172 (i.e., the heavily doped region of the substrate 110 doped withimpurities of the first conductive type).

Alternatively, at least one of the front electrode part 140 and the backelectrode part 150 may be formed through a screen printing method usingan Ag paste containing a glass frit or an Al paste containing the glassfrit. In this instance, the glass frit may pass through the firstdielectric layer part 130 or the second dielectric layer part 190 andmay abut on the emitter region 121 or the back surface field regions172.

Accordingly, at least one of components of the glass frit is detected ina contact portion between the front electrode part 140 and the emitterregion 121 or a contact portion between the back electrode part 150 andthe back surface field regions 172. For example, at least one of lead(Pb)-based material such as PbO, bismuth (Bi)-based material such asBi₂O₃, aluminum (Al)-based material such as Al₂O₃, boron (B)-basedmaterial such as B₂O₃, tin (Sn)-based material, zinc (Zn)-based materialsuch as ZnO, titanium (Ti)-based material such as TiO, and phosphorus(P)-based material such as P₂O₅ contained in the glass frit may bedetected.

On the other hand, when at least one of the front electrode part 140 andthe back electrode part 150 is formed using the plating method, thecomponent of the glass frit is not detected between the substrate 110(i.e., the emitter region 121) and the front electrode part 140including the front electrodes 141 and the front bus bars 142 andbetween the substrate 110 (i.e., the back surface field regions 172) andthe back electrode part 150 including the back electrodes 151 and theback bus bars 152.

As described above, when at least one of the front electrode part 140and the back electrode part 150 has the multi-layered structure, thelower layer, the middle layer, and the upper layer are sequentiallyformed using the plating method to have a desired thickness.

In the embodiment of the invention, the number of front electrodes 141,the number of front bus bars 142, the number of back electrodes 151, andthe number of back bus bars 152 may vary, if desired or necessary.

Each front bus bar 142 and each back bus bar 152 respectively collectcarriers from the emitter region 121 and the back surface field regions172 and also respectively output carriers collected by the frontelectrodes 141 and carriers collected by the back electrodes 151 to theexternal device.

In another embodiment, at least one of the front bus bar 142 and theback bus bar 152 may be positioned directly on at least one of the firstdielectric layer part 130 and the second dielectric layer part 190 andmay abut on at least one of the first dielectric layer part 130 and thesecond dielectric layer part 190.

As described above, because each of the front surface and the backsurface of the substrate 110 has the first and second textured surfaces,a surface area of the substrate 110 increases.

Hence, an area of the emitter region 121 contacting each front electrode141 and an area of the back surface field region 172 contacting eachback electrode 151 increase. Therefore, even if a width W11 of eachfront electrode 141 and a width W12 of each back electrode 151 decrease,a contact area between the emitter region 121 and the front electrode141 and a contact area between the back surface field region 172 and theback electrode 151 may not decrease.

As a result, even if the width W11 of each front electrode 141 and thewidth W12 of each back electrode 151 decrease, an amount of carriersmoving from the emitter region 121 to the front electrodes 141 and anamount of carriers moving from the back surface field regions 172 to theback electrodes 151 may not decrease.

In the embodiment of the invention, the width W11 of each frontelectrode 141 and the width W12 of each back electrode 151 may be about40 μm to 50 μm.

As described above, because formation areas of the front electrodes 141and the back electrodes 151 to prevent or reduce the incidence of lightat the front surface and the back surface of the substrate 110 arereduced, an amount of light incident on the front surface and the backsurface of the substrate 110 increases.

However, a moving distance of carriers moving to the front electrodes141 along the emitter region 121 and a moving distance of carriersmoving to the back electrodes 151 along the back surface field regions172 increase because of the first and second textured surfaces of thesubstrate 110.

Accordingly, in the embodiment of the invention, a distance D11 betweenthe two adjacent front electrodes 141 and a distance D12 between the twoadjacent back electrodes 151 may decrease, so as to compensate for anincrease in the moving distance of carriers resulting from an increasein a surface area of the emitter region 121 and a surface area of theback surface field regions 172.

For example, the distance D11 between the two adjacent front electrodes141 and the distance D12 between the two adjacent back electrodes 151may be equal to or greater than about 1.5 mm and less than about 2.0 mm.

As described above, because the width W11 of each front electrode 141and the width W12 of each back electrode 151 decrease, an incident areaof light at the front surface and the back surface of the substrate 110does not decrease even if the distances D11 and D12 increases.

An operation of the solar cell having the above-described structure isdescribed below.

When light irradiated to the solar cell is incident on the substrate 110through at least one of the first dielectric layer part 130 and thesecond dielectric layer part 190, electrons and holes are generated inthe substrate 110 by light energy produced based on the incident light.

In this instance, because a reflection loss of the light incident on thesubstrate 110 is reduced by the first and second textured surfaces ofthe substrate 110, the first dielectric layer part 130, and the seconddielectric layer part 190, an amount of light incident on the substrate110 increases.

The electrons move to the n-type semiconductor part (for example, thesubstrate 110) and the holes move to the p-type semiconductor part (forexample, the emitter region 121) by the p-n junction of the substrate110 and the emitter region 121.

The holes moving to the emitter region 121 are collected by the frontelectrodes 141 and the front bus bars 142 and then move along the frontbus bars 142. The electrons moving to the substrate 110 pass through theback surface field regions 172, are collected by the back electrodes 151and the back bus bars 152, and move along the back bus bars 152.

When the front bus bars 142 of one solar cell are connected to the backbus bars 152 of another solar cell adjacent to the one solar cell usingelectric wires such as a conductive film, current flows therein tothereby enable use of the current for electric power.

As described above, each of the front surface and the back surface ofthe substrate 110, on which light is incident, has the double texturingstructure, the incident area of the substrate 110 increases. Further, areflection amount of light decreases due to a reflection operation usingthe first and second protrusions 11 and 111, and an amount of lightincident on the substrate 110 increases.

The efficiency of the solar cell is improved by the anti-reflectioneffect using the refractive indexes of the first dielectric layer part130 and the second dielectric layer part 190, the field passivationeffect using the fixed charges, and the surface passivation effect usinghydrogen (H) or oxygen (O).

A method for manufacturing the solar cell according to the embodiment ofthe invention is described below with reference to FIGS. 4A to 4H.

First, a substrate 110 is generally manufactured by slicing a siliconblock or an ingot using a blade or a multi-wire saw. When the siliconblock or the ingot is sliced, a mechanical damage layer is formed in thesubstrate 110.

Accordingly, a wet etching process is performed to remove the mechanicaldamage layer, so as to prevent a reduction in characteristics of thesolar cell resulting from the mechanical damage layer of the substrate110. In this instance, as shown in FIG. 4A, a first textured surfaceincluding a plurality of first protrusions 11 is formed on at least onesurface of the substrate 110.

The first textured surface may be formed through the wet etchingprocess. The wet etching process may use at least one of an acidchemical and a basic chemical.

An example of a process for forming the first textured surface isdescribed below.

First, the wet etching process using the basic chemical is performed toetch at least one surface of the substrate 110. Examples of the basicchemical include potassium hydroxide (KOH), isopropyl alcohol (IPA), orother organic additives.

As described above, when the surface of the substrate 110 is etchedusing the basic chemical, the surface of the substrate 110 is texturedto have the first textured surface including the plurality of firstprotrusions 11.

According to the method described above, after the plurality of firstprotrusions 11 are formed, a second textured surface including aplurality of second protrusions 111 is formed on the surface of each ofthe plurality of first protrusions 11 using a dry etching method such asa reaction ion etching (RIE) method.

FIG. 4B shows the plurality of second protrusions 111 formed on thesurface of each first protrusion 11.

Each of the plurality of second protrusions 111 has the size of about300 nm to 600 nm. The plurality of second protrusions 111 aredistributed on the surface of each first protrusion 11, so that in avertical cross section of the second protrusions 111, a ratio a1/b1 of alength a1 of a virtual line connecting vertexes of the secondprotrusions 111 to a length b1 of a straight line connecting a startpoint and a finish point of the virtual line is about 1.1 to 1.3.

In the embodiment of the invention, an etching gas used in the reactionion etching method may be a mixture of SF₆ and Cl₂.

After the second textured surface is formed, a process for removing aresidue remaining in the surface of the substrate 110 is performed.

When the second protrusions 111 are formed using the reaction ionetching method, a large amount of impurities are adsorbed on the surfaceof the substrate 110 as shown in FIGS. 6A and 6B.

More specifically, FIG. 6A is a photograph of the first protrusion 11having the second protrusions 111 magnified by a microscope atmagnification capacity of 25,000 times. FIG. 6B is a photograph of thefirst protrusion 11 having the second protrusions 111 magnified by amicroscope at magnification capacity of 90,000 times.

A damaged layer resulting from plasma was formed on the surface of eachof the second protrusions 111 formed using the reaction ion etchingmethod.

In a comparative example, a first cleansing process and a secondcleansing process using an acid chemical were sequentially performed soas to remove the damaged layer.

An acid chemical obtained by mixing sulfuric acid (H₂SO₄) and hydrogenperoxide in a ratio of 1:1 to 4:1 was used in the first cleansingprocess. An acid chemical obtained by mixing ultrapure water, hydrogenchloride (HCl), and hydrogen peroxide in a ratio of 5:1:1 was used inthe second cleansing process.

FIGS. 7A and 7B are photographs of minute protrusions taken by amicroscope after a first cleansing process using an acid chemicalobtained by mixing sulfuric acid (H₂SO₄) and hydrogen peroxide in aratio of 1:1 to 4:1 and a second cleansing process using an acidchemical obtained by mixing ultrapure water, hydrogen chloride (HCl),and hydrogen peroxide in a ratio of 5:1:1 are performed. Morespecifically, FIG. 7A is a photograph of the first protrusion 11 havingthe second protrusions 111 magnified by the microscope at magnificationcapacity of 25,000 times. FIG. 7B is a photograph of the firstprotrusion 11 having the second protrusions 111 magnified by themicroscope at magnification capacity of 90,000 times.

The impurities generated, when the first and second cleansing processesusing the acid chemical were performed after forming the secondprotrusions 111 as shown in FIGS. 7A and 7B, were greatly removed ascompared to FIGS. 6A and 6B.

However, even after the first and second cleansing processes using theacid chemical were performed, the damaged layers of the secondprotrusions 111 were not efficiently removed, and the impurities werenot completely removed.

In the embodiment of the invention, a basic chemical instead of the acidchemical is used in the first cleansing process, so as to solve theabove problem.

This is described in detail below with reference to FIG. 5.

A cleansing method according to the embodiment of the invention mayinclude a first cleansing process which removes surface damage portionsof a plurality of minute protrusions using a basic chemical and removesimpurities adsorbed on the surfaces of the minute protrusions, a processfor cleansing the surfaces of the minute protrusions using a dilutedacid chemical, a second cleansing process for removing impuritiesremaining or again adsorbed on the surfaces of the minute protrusionsusing an acid chemical after the first cleansing process, and a processfor again cleansing the surfaces of the minute protrusions using adiluted acid chemical.

In the embodiment disclosed herein, the process for cleansing thesurfaces of the minute protrusions using the diluted acid chemical,i.e., at least one of the cleansing process performed between the firstcleansing process and the second cleansing process and the cleansingprocess performed after the second cleansing process may be omitted.

The first cleansing process may use a basic chemical obtained by mixingultrapure water, ammonium hydroxide (NH₄OH), and hydrogen peroxide(H₂O₂) in a ratio of 5:1:1. The first cleansing process using the basicchemical may be performed at a temperature equal to or lower than about70° C. for about 5 to 10 minutes.

Potassium hydroxide (KOH) having an etching performance better thanammonium hydroxide (NH₄OH) may be used in the first cleansing processinstead of ammonium hydroxide (NH₄OH). Basic materials, having hydroxylradical (OH), other than ammonium hydroxide (NH₄OH) and potassiumhydroxide (KOH) may be used in the first cleansing process.

When the first cleansing process using the basic chemical is performed,the surface of the second protrusion is removed through the etching of athin thickness, and impurities adsorbed on the surface of the secondprotrusion are removed.

The second cleansing process may use an acid chemical obtained by mixingultrapure water, hydrogen chloride (HCl), and hydrogen peroxide (H₂O₂)in a ratio of 5:1:1. The second cleansing process using the acidchemical may be performed at a temperature equal to or lower than about70° C. for about 5 to 10 minutes.

The second cleansing process using the acid chemical efficiently removesimpurities again adsorbed on the surfaces of the second protrusionsafter the first cleansing process.

The diluted acid chemical may be obtained by mixing ultrapure water andhydrogen fluoride (HF) in a ratio of 10:1 to 7:1. The cleansing processusing the diluted acid chemical may be performed at the normaltemperature for about 5 to 10 minutes.

FIGS. 8A and 8B are photographs of minute protrusions taken by amicroscope after a first cleansing process using a basic chemical and asecond cleansing process using an acid chemical are performed. Morespecifically, FIG. 8A is a photograph of the first protrusion 11 havingthe second protrusions 111 magnified by the microscope at magnificationcapacity of 25,000 times. FIG. 8B is a photograph of the firstprotrusion 11 having the second protrusions 111 magnified by themicroscope at magnification capacity of 90,000 times.

The impurities generated, when the first cleansing process using thebasic chemical and the second cleansing process using the acid chemicalwere performed after forming the second protrusions 111 as shown inFIGS. 8A and 8B, were more efficiently removed as compared to FIGS. 7Aand 7B.

The following Table 1 indicates characteristics of the solar cellmeasured after performing the cleansing process according to each of theembodiment of the invention and the comparative example. In thefollowing Table 1, the cleansing process according to the comparativeexample used the acid chemical in both the first and second cleansingprocesses. A first example of the cleansing process according to theembodiment of the invention used ammonium hydroxide (NH₄OH) in the firstcleansing process using the basic chemical, and a second example of thecleansing process according to the embodiment of the invention usedpotassium hydroxide (KOH) in the first cleansing process using the basicchemical.

TABLE 1 Open-circuit Short circuit voltage current density Fill factorEfficiency Comparative 649 38.6 79.5 19.9 example First example 650 38.680.0 20.1 embodiment Second example 650 38.6 80.0 20.1 embodiment

As indicated by the above Table 1, the characteristics of the solar cellin the first and second examples according to the embodiment of theinvention were greatly improved, as compared to the comparative exampleusing the acid chemical in both the first and second cleansingprocesses.

So far, the cleansing method according to the embodiment of theinvention including the first cleansing process using the basic chemical(for example, a moisture of ultrapure water, potassium hydroxide, andhydrogen peroxide), the cleansing process using the diluted acidchemical (for example, a moisture of ultrapure water and hydrogenfluoride), the second cleansing process using the acid chemical (forexample, a moisture of ultrapure water, hydrogen chloride, and hydrogenperoxide), and the cleansing process using the diluted acid chemical(for example, a moisture of ultrapure water and hydrogen fluoride) wasdescribed.

Alternatively, the basic chemical used in the first cleansing processmay not contain hydrogen peroxide. In other words, the basic chemicalused in the first cleansing process may be formed by mixing a basicmaterial having hydroxyl radical (OH), for example, potassium hydroxideor ammonium hydroxide and ultrapure water.

In another example of the cleansing method, the second cleansing processusing the acid chemical and the cleansing process using the diluted acidchemical may be integrated into one process and may be performed as oneprocess. Namely, the two cleansing processes may be reduced to oneprocess.

When the second cleansing process using the acid chemical and thecleansing process using the diluted acid chemical are integrated intoone process and are performed as one process, the acid chemical may beformed by mixing ultrapure water, hydrogen chloride, and hydrogenfluoride.

Accordingly, in this instance, the cleansing method according to theembodiment of the invention may include the first cleansing processusing the basic chemical (for example, the moisture of ultrapure water,potassium hydroxide, and hydrogen peroxide), the cleansing process usingthe diluted acid chemical (for example, the moisture of ultrapure waterand hydrogen fluoride), and the second cleansing process using the acidchemical (for example, the moisture of ultrapure water, hydrogenchloride, and hydrogen fluoride).

In another example of the cleansing method, the cleansing process usingthe diluted acid chemical, the second cleansing process using the acidchemical, and the cleansing process using the diluted acid chemical maybe integrated into one process and may be performed as one process.Namely, the three cleansing processes may be reduced to one process.

When the cleansing process using the diluted acid chemical, the secondcleansing process using the acid chemical, and the cleansing processusing the diluted acid chemical are integrated into one process and areperformed as one process, the acid chemical may be formed by mixingultrapure water, hydrogen chloride, and hydrogen fluoride.

Accordingly, in this instance, the cleansing method according to theembodiment of the invention may include the first cleansing processusing the basic chemical (for example, the moisture of ultrapure water,potassium hydroxide, and hydrogen peroxide) and the second cleansingprocess using the acid chemical (for example, the moisture of ultrapurewater, hydrogen chloride, and hydrogen fluoride). After the surface ofthe semiconductor substrate 110 is cleansed through the variouscleansing methods described above, an emitter region 121 is formed at afront surface of the substrate 110 as shown in FIG. 4C.

More specifically, as shown in FIG. 4C, the emitter region 121 may beformed by implanting first impurities of a corresponding conductive type(i.e., a second conductive type) into the front surface of the substrate110 using an ion implantation method and then performing an activationprocess.

In this instance, the conductive type of the emitter region 121 may bethe second conductive type (for example, p-type) opposite a firstconductive type of the substrate 110. In the embodiment of theinvention, the first impurities may use boron (B).

Accordingly, the first impurities of the second conductive type areimplanted into the exposed front surface of the substrate 110 to form animpurity region, i.e., a first impurity region 120 of the secondconductive type at the front surface of the substrate 110.

Subsequently, second impurities of a corresponding conductive type(i.e., the first conductive type) (for example, n-type) are implantedinto a back surface of the substrate 110 using the ion implantationmethod to form a second impurity region 170 of the first conductive typeat the back surface of the substrate 110. In the embodiment of theinvention, the second impurities may use phosphorus (P).

A mask to implant the impurities only into a desired region of each ofthe front surface and the back surface of the substrate 110 may be usedin the ion implantation process for forming the first and secondimpurity regions 120 and 170.

For example, the mask positioned on the front surface of the substrate110 may block only an edge of the front surface of the substrate 110 andmay expose a remaining portion of the front surface of the substrate 110except the edge. Further, the mask positioned on the back surface of thesubstrate 110 may expose an edge of the back surface of the substrate110 and a formation area of back electrodes and back bus bars and mayblock a remaining portion of the back surface of the substrate 110.

Ion implantation energy for implanting first impurity ions and secondimpurity ions into the substrate 110 may be about 1 keV to 20 keV. Anion implantation depth may be determined depending on the ionimplantation energy.

Thus, ion implantation energy for the first impurity region 120 may bedifferent from ion implantation energy for the second impurity region170.

For example, ion implantation energy used to implant ions of p-typeimpurities into the substrate 110 may be greater than ion implantationenergy used to implant ions of n-type impurities into the substrate 110.

In the embodiment of the invention, formation order of the firstimpurity region 120 and the second impurity region 170 may vary. Thefirst impurity region 120 and the second impurity region 170 may beformed in the same chamber or respective chambers.

Next, as shown in FIG. 4D, after the first impurity region 120 and thesecond impurity region 170 are formed, a thermal process is performed onthe substrate 110 in an atmosphere of nitrogen (N₂) or oxygen (O₂).

Hence, the first and second impurity regions 120 and 170 are completelyactivated. As a result, the first impurity region 120 forms an emitterregion 121 positioned at the front surface of the substrate 110, andthus the emitter region 121 and the substrate 110 form a p-n junction.Further, the second impurity region 170 forms a plurality of backsurface field regions 172 positioned at the back surface of thesubstrate 110.

In other words, the p-type impurities and the n-type impuritiesimplanted into the substrate 110 respectively form the first impurityregion 120 and the second impurity region 170 in an interstitial state.However, when the first and second impurity regions 120 and 170 areactivated through the thermal process, a state of impurities is changedfrom the interstitial state to a substitutional state. Hence, siliconand ions of p-type and n-type impurities are rearranged. As a result,the first impurity region 120 and the second impurity region 170respectively serve as the p-type emitter region 121 and the n-type backsurface field region 172.

Solubility of boron (B) used to form the emitter region 121 is less thansolubility of phosphorus (P) used to form the back surface field regions172. Therefore, it is preferable, but not required, that an activationtemperature of the first and second impurity regions 120 and 170 isdetermined based on the first impurity region 120, so as to stablyactivate the first impurity region 120.

Thus, in the embodiment of the invention, the activation temperature ofthe first and second impurity regions 120 and 170 may be a temperaturecapable of stably activating the first impurity region 120, for example,about 1,000° C. to 2,000° C. Further, time required in the thermalprocess may be about 20 minutes to 60 minutes.

In the embodiment of the invention, because the activation process isperformed at a high temperature equal to or higher than about 1,000° C.capable of stably activating boron (B) (i.e., the first impurity region120), the first impurity region 120 as well as the second impurityregion 170 are stably activated. Hence, the emitter region 121 and theback surface field regions 172 are smoothly formed.

Furthermore, because the thermal process is performed at the hightemperature equal to or higher than about 1,000° C., a damaged portiongenerated in the ion implantation process for the first and secondimpurity regions 120 and 170 is recrystallized. Hence, the damagedportion generated in the ion implantation process is recovered without aseparate removal process using the wet etching method, etc.

So far, the embodiment of the invention described that the emitterregion 121 and the back surface field regions 172 are formed through theion implantation method, as an example. However, the emitter region 121and the back surface field regions 172 may be formed through a typicalthermal diffusion method.

Next, as shown in FIG. 4E, a first dielectric layer 191 formed ofhydrogenated silicon nitride (SiNx:H) is formed on the back surface ofthe substrate 110.

The first dielectric layer 191 may be formed using a deposition methodsuch as a plasma enhanced chemical vapor deposition (PECVD) method. Inthe embodiment of the invention, the first dielectric layer 191 may havea thickness of about 70 nm to 100 nm.

Next, as shown in FIG. 4F, second dielectric layers 131 formed ofaluminum oxide (Al₂O₃) are respectively formed on the emitter region 121positioned at the front surface of the substrate 110 and the firstdielectric layer 191 positioned on the back surface of the substrate110.

The second dielectric layer 131 may be formed using the PECVD method, anatomic layer deposition (ALD) method, etc.

When the second dielectric layer 131 is formed using the PECVD method,the layer is stacked only in a portion exposed by a process gas.Therefore, the second dielectric layer 131 formed of aluminum oxide maybe formed on each of the front surface and the back surface of thesubstrate 110 through the separate PECVD method.

In this instance, the second dielectric layers 131 respectivelypositioned on the front surface and the back surface of the substrate110 may be formed under the same process conditions and thus may havethe same characteristics. Alternatively, the second dielectric layers131 may be formed under different process conditions and thus may havedifferent characteristics.

On the other hand, when the second dielectric layers 131 are formedusing the ALD method, the second dielectric layers 131 may be formed onthe back surface and the lateral surface as well as the front surface ofthe substrate 110 through one stacking process.

Thus, the ALD process may be performed once to simultaneously form thesecond dielectric layers 131 on the front surface and the back surfaceof the substrate 110. In this instance, because the second dielectriclayers 131 respectively positioned on the front surface and the backsurface of the substrate 110 are formed under the same processconditions, the second dielectric layers 131 have the samecharacteristics.

In the embodiment of the invention, the second dielectric layer 131 mayhave a thickness of about 5 nm to 15 nm.

When the second dielectric layer 131 is formed, a second dielectriclayer part 190 including the first dielectric layer 191 and the seconddielectric layer 131 is formed on the back surface of the substrate 110.

Next, as shown in FIG. 4G, a third dielectric layer 132 formed ofhydrogenated silicon nitride (SiNx:H) is formed on the second dielectriclayer 131 on the front surface of the substrate 110 using the PECVDmethod at a thickness of about 70 nm to 100 nm.

Hence, a first dielectric layer part 130 including the second dielectriclayer 131 and the third dielectric layer 132 is formed on the frontsurface of the substrate 110.

As described above, the first dielectric layer part 130 is formed on thefront surface of the substrate 110, and the second dielectric layer part190 is formed on the back surface of the substrate 110. Afterward, afront electrode part 140 including a plurality of front electrodes 141and a plurality of front bus bars 142, which pass through the firstdielectric layer part 130 and contact the emitter region 121 underlyingthe first dielectric layer part 130, is formed. Further, a backelectrode part 150 including a plurality of back electrodes 151 and aplurality of back bus bars 152, which pass through the second dielectriclayer part 190 and contact the back surface field regions 172 underlyingthe second dielectric layer part 190, is formed.

An example of a method for forming the front electrode part 140 and theback electrode part 150 is described below with reference to FIG. 4H.

For example, as shown in FIG. 4H, a laser beam is selectively irradiatedonto each of the front surface and the back surface of the substrate 110to form a plurality of first openings 181 and a plurality of secondopenings 182 at positions to form the front electrode part 140 and theback electrode part 150.

The plurality of first openings 181 pass through the first dielectriclayer part 130 and expose the emitter region 121 underlying the firstdielectric layer part 130. The plurality of second openings 182 passthrough the second dielectric layer part 190 and expose the back surfacefield regions 172 underlying the second dielectric layer part 190.

The plurality of first openings 181 are used to form the plurality offront electrodes 141 and the plurality of front bus bars 142. In thisinstance, a width of the first opening 181 for each front electrode 141is less than a width of the first opening 181 for each front bus bar142.

The plurality of second openings 182 are used to form the plurality ofback electrodes 151 and the plurality of back bus bars 152. In thisinstance, a width of the second opening 182 for each back electrode 151is less than a width of the second opening 182 for each back bus bar152.

The number of first openings 181 for the front electrodes 141 may beless than the number of second openings 182 for the back electrodes 151.Hence, a distance between the two adjacent first openings 181 may begreater than a distance between the two adjacent second openings 182.

The first openings 181 for the front bus bars 142 may be positionedopposite the second openings 182 for the back bus bars 152 with thesubstrate 110 between them.

Subsequently, the front electrode part 140 including the frontelectrodes 141 and the front bus bars 142 is formed on the emitterregion 121 exposed by the first openings 181 using a plating method suchas an electroplating method and an electroless plating method. In thesame manner as the front electrode part 140, the back electrode part 150including the back electrodes 151 and the back bus bars 152 is formed onthe back surface field regions 172 exposed by the second openings 182using the plating method.

In another embodiment, the front electrodes 141, the front bus bars 142,the back electrodes 151, and the back bus bars 152 may be formed bycoating a metal paste containing a metal material, for example, silver(Ag) on the first and second openings 181 and 182 using a screenprinting method, etc. and then performing a thermal process on the metalpaste at a desired temperature.

In another embodiment, the front electrodes 141, the front bus bars 142,the back electrodes 151, and the back bus bars 152 may be formed bycoating and drying a metal paste containing silver (Ag) or a metal pastecontaining silver (Ag) and aluminum (Al) on each of the first dielectriclayer part 130 and the second dielectric layer part 190 and thenperforming a thermal process on the metal paste.

In this instance, the front electrode part 140 have to pass through thefirst dielectric layer part 130, and the back electrode part 150 have topass through the second dielectric layer part 190.

Accordingly, the metal paste may contain a material, for example, PbOfor etching the first dielectric layer part 130 and the seconddielectric layer part 190. An amount and a kind of the etching materialcontained in the metal paste may be determined depending on a thicknessor a material of the first dielectric layer part 130 and the seconddielectric layer part 190.

Accordingly, when the thermal process is performed on the metal pastewhich is coated and dried on the first dielectric layer part 130 and thesecond dielectric layer part 190, the metal paste passes through thefirst dielectric layer part 130 and the second dielectric layer part 190and is chemically bonded to the emitter region 121 and the back surfacefield regions 172. Hence, the front electrode part 140 electrically andphysically connected to the emitter region 121 and the back electrodepart 150 electrically and physically connected to the back surface fieldregions 172 are formed.

Alternatively, when at least one of the first dielectric layer part 130and the second dielectric layer part 190 is omitted, a metal pasteforming an electrode part, which does not need to pass through the firstdielectric layer part 130 and the second dielectric layer part 190, maynot contain an etching material or may contain the etching material tothe extent that it does not affect the pass of the first dielectriclayer part 130 and the second dielectric layer part 190.

The first dielectric layer part 130 may additionally include ahydrogenated silicon oxide layer, which is positioned directly on thethird dielectric layer 132 and is formed of hydrogenated silicon oxide(SiOx:H). Further, the second dielectric layer part 190 on the backsurface of the substrate 110 may additionally include a hydrogenatedsilicon oxide layer, which is positioned between the first dielectriclayer 191 formed of hydrogenated silicon nitride (SiNx:H) and the seconddielectric layer 131 formed of aluminum oxide (Al₂O₃) and is formed ofhydrogenated silicon oxide (SiOx:H).

The hydrogenated silicon oxide layer prevents hydrogen (H), which existsin the third dielectric layer 132 and the first dielectric layer 191underlying the hydrogenated silicon oxide layer to perform the surfacepassivation function, from moving to the opposite side of the substrate110. Hence, the passivation effect and the anti-reflection effect at thesurface of the substrate 110 are further improved.

A silicon oxide layer formed of silicon oxide (SiOx) may be additionallyformed between the emitter region 121 and the first dielectric layerpart 130 and between the back surface of the substrate 110 and the firstdielectric layer 191.

The silicon oxide layer may suppress a blistering phenomenon generatedwhen an aluminum oxide (Al₂O₃) layer, etc., is formed on a natural oxidelayer, and may further improve the passivation effect.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A method for manufacturing a solar cellcomprising: performing a dry etching process to form a textured surfaceincluding a plurality of minute protrusions on a first surface of asemiconductor substrate; performing a first cleansing process forremoving damaged portions of surfaces of the minute protrusions using abasic chemical and removing impurities adsorbed on the surfaces of theminute protrusions; performing a second cleansing process for removingimpurities remaining or again adsorbed on the surfaces of the minuteprotrusions using an acid chemical after performing the first cleansingprocess; and forming an emitter region at the first surface of thesemiconductor substrate.
 2. The method of claim 1, wherein the dryetching process includes a reaction ion etching method.
 3. The method ofclaim 1, wherein the basic chemical is formed by mixing ultrapure waterand a basic material having a hydroxyl radical (OH).
 4. The method ofclaim 3, wherein the basic chemical additionally contains hydrogenperoxide.
 5. The method of claim 3, wherein the basic material havingthe hydroxyl radical includes a potassium hydroxide solution or anammonium hydroxide solution.
 6. The method of claim 1, wherein the acidchemical is formed by mixing ultrapure water, hydrogen chloride, andhydrogen peroxide.
 7. The method of claim 6, further comprising againcleansing the surfaces of the minute protrusions using a diluted acidchemical between the first cleansing process and the second cleansingprocess.
 8. The method of claim 7, wherein the diluted acid chemical isformed by mixing ultrapure water and hydrogen fluoride.
 9. The method ofclaim 6, further comprising, after the second cleansing process, againcleansing the surfaces of the minute protrusions using a diluted acidchemical.
 10. The method of claim 9, wherein the diluted acid chemicalis formed by mixing ultrapure water and hydrogen fluoride.
 11. Themethod of claim 1, wherein the acid chemical is formed by mixingultrapure water, hydrogen chloride, and hydrogen fluoride.
 12. Themethod of claim 1, wherein the forming of the emitter region includesinjecting impurities of a first conductive type into the first surfaceof the semiconductor substrate using an ion implantation method or athermal diffusion method.
 13. The method of claim 12, furthercomprising: forming a second textured surface including a plurality ofminute protrusions on a second surface opposite the first surface of thesemiconductor substrate; and locally forming a back surface field regionat the second surface of the semiconductor substrate.
 14. The method ofclaim 13, wherein the forming of the back surface field region includesinjecting impurities of a second conductive type opposite the firstconductive type into the second surface of the semiconductor substrateusing the ion implantation method or the thermal diffusion method. 15.The method of claim 14, further comprising: forming a first dielectriclayer on the second surface of the semiconductor substrate;simultaneously forming a second dielectric layer on the emitter regionand on the first dielectric layer positioned on the second surface ofthe semiconductor substrate; forming a third dielectric layer on thesecond dielectric layer positioned on the emitter region; and forming afirst electrode part connected to the emitter region and a secondelectrode part connected to the back surface field region.
 16. Themethod of claim 15, wherein the first dielectric layer and the thirddielectric layer are formed by depositing hydrogenated silicon nitrideat a thickness of about 70 nm to 100 nm, wherein the second dielectriclayer is formed by depositing aluminum oxide at a thickness of about 5nm to 15 nm, aluminum oxide being deposited using an atomic layerdeposition method.
 17. The method of claim 15, wherein the back surfacefield region is formed in the same pattern as a plurality of fingerelectrodes of the second electrode part.